In the prior art a brake master cylinder is known, which comprises two piston assemblies with substantially identical structure, namely, a primary piston assembly and a secondary piston assembly. Each piston assembly comprises a piston, a piston stop and a spring acting between the piston and the piston stop. Usually, the piston stop is assembled on the piston by means of threaded connection or riveting. Therefore, some extra parts such as a screw, a pin, a spring seat or the like are needed when the brake master cylinder is assembled. Such brake master cylinder is structurally complicated and has many parts. In most cases, each piston assembly of the brake master cylinder needs 4-5 parts. Such a complicated structure causes the manufacturing cost and assembling cost of the brake master cylinder very high.
A piston-type brake master cylinder for a vehicle is known from the patent document ZL200820186125.8 and comprises a master cylinder block, a first piston and a second piston, wherein a spring is disposed in an inner cavity of a piston stop, namely, disposed in the piston stop. However, if a size of a bore for forming the inner cavity is too small, an enough mounting space cannot be provided for the spring, so the spring cannot be encapsulated in the piston stop. In addition, the clearance between an outer diameter of the spring and an inner diameter of the piston stop is very small, the outer-diameter portion of the spring might contact the inner-diameter portion of the piston stop, namely, it might cause plastic to spring contact that could produce plastic debris and thereby affect sealing effect of a sealing member and cause hydraulic leakage. Therefore, the assembling of the spring is limited by the bore diameter of the piston stop, and the spring cannot be applied to a brake master cylinder assembly with a small bore diameter.
In addition, a piston stop sleeved around a positioning pin at a center of the piston is known from the prior art. Although there are a plurality of elongate holes on a circumferential wall of the piston stop, the bottom portion of the piston stop is a closed circumferential portion and it cannot open and deform when the piston stop snaps into the piston. Therefore, the piston stop almost cannot be installed. Even if it is installed forcedly, the piston stop scratches with the piston and thereby produces debris due to too tight contact between the piston stop and the piston upon assembling, thereby affecting the sealing action of the sealing member. Besides, the piston stop might break duo to forced assembling.